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-   -   Question about transitional model (http://www.cfd-online.com/Forums/cfx/114017-question-about-transitional-model.html)

Anna Tian March 3, 2013 19:43

Question about transitional model
 
Hi,

For my case, turbulent flow transfer to laminar in a narrow rectangle pipe. 5% of the flow is turbulent, 15% is transitional, 80% is laminar. I care about the overall total pressure drop prediction. 5% of the total pressure drop is in turbulent area. 15% is in transitional area. And 80% is in laminar area.

Shall I still use the transitional model for this case? How's the transitional model work for the laminar part of the flow? Or I'd better switch to laminar model?

Thanks!

ghorrocks March 4, 2013 05:39

The transitional model is for laminar to turbulent transition, not the other way around. The model is not suitable for your laminarisation model.

Two options:
1) Run using a turbulence model which degenerates to zero turbulence nicely, such as SST - this is a good option if the important stuff happens in the turbulent region. OR
2) Run using a laminar flow model. This is a god option if the laminar bit is most important.

Anna Tian March 4, 2013 07:31

Quote:

Originally Posted by ghorrocks (Post 411299)
The transitional model is for laminar to turbulent transition, not the other way around. The model is not suitable for your laminarisation model.

Two options:
1) Run using a turbulence model which degenerates to zero turbulence nicely, such as SST - this is a good option if the important stuff happens in the turbulent region. OR
2) Run using a laminar flow model. This is a god option if the laminar bit is most important.


Why the transitional model can't be used the other way around? I read the the transitional model part of CFX tutorial.

https://www.sharcnet.ca/Software/Flu.../i1324236.html

It doesn't tell that it's only for laminar-turbulent transition.

Thanks.

ghorrocks March 4, 2013 17:45

You have to read the research papers which describe this model in more detail. The model was tuned to model lam to turb transition in airfoils and similar geometries, where the transition occurs in the boundary layer and spreads from there. It has not been tuned to handle lam-turb transition in bulk flows.


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